In the past, many designs have been engineered to address the problem of translating the concept of continuously casting a molten metal using a moving mold incorporated within an operable and reliable machine. Towards this end, about 20 years ago, Lauener disclosed a continuous casting machine in U.S. Pat. No. 3,570,586, which used multiple mold block assemblies to form a continuous mold cavity from which metal emerged as a slab casting. For this reason, the machine is referred to as a "block caster" (or "slab caster"). Limited commercial success of the Lauener machine has, over the years spurred much effort towards refining the concept to build a machine to produce a quality slab casting, reliably and economically.
In a block caster, a pair of synchronously driven endless trains of mold block assemblies ("casting trains") travelling in paths which resemble loops, define a substantially linear mold cavity having open ends at each end, when the opposed faces of mold blocks in the mold block assemblies come together in spaced-apart relationship, facing each other, along the linear portions of the loops. Each loop has its linear portion connected by upper and lower arcuate portions or "bends" which complete the loops, and the mold block assemblies are endlessly interconnected and oppositely disposed relative to one another in the adjacent linear portions of the loops. The mold cavity is preferably defined in conjunction with "side dams" which together confine the molten metal in the moving mold cavity.
A casting nozzle is inserted near one open end, referred to as the "molten end", to supply the mold cavity with molten metal from a tundish. The molten metal cools as it progresses with the mold block assemblies until the slab emerges from the other open end, referred to as the "solid end". The longitudinal direction in which the molten metal is cast is referred to as the "casting direction" or the x-axis. The lateral direction, transverse to that in which the metal is cast, is referred to as the "transverse direction" or y-axis; and the vertically spaced-apart distance of the mold block faces which define the thickness of the cast slab is said to be in the vertical direction, or z-axis.
As one might expect, to cast a slab with rectangular cross-section, it is essential that the faces of the mold blocks defining the upper surface of the slab be in the same plane, and that the faces of those mold blocks defining the lower surface of the slab be in the same plane. When any portion of the face of a mold block is displaced from its original planar configuration, the slab will not have planar upper and lower surfaces. Depending upon the type of displacement, the surfaces will be arcuate, rippled or striated.
Portions of each mold block face in a zone near the block's surface are displaced because these portions get progressively hotter as they move through a primary process zone in contact with molten metal at the molten end. These portions near the surface then progressively cool as heat is transferred into the body of the block and the slab begins to cool as it reaches the solid end. The mold blocks are necessarily distorted due to temperature differences associated with the temperature gradients which are three-dimensionally distributed through the mold block. As the mold blocks cool and the molten metal soldifies, their original dimensions begin to be restored, returning to normal when sufficiently cooled. It is the distortion of the mold blocks during operation which must be dealt with. The choice is either to counteract the distortion, or to compensate for it. How one designs and constructs a block caster depends on the choice.
Under actual operating conditions, the distortions of the mold blocks are such that they exert enormous pressure against contiguous mold blocks forcing the metal of the blocks out of their planar conformance in edge-abutting protuberances, referred to as "bumps" described in greater detail in FIGS. 10-12. These bumps interfere with the smoothly planar definition of the surfaces of the upper and lower series of mold block faces. As a result, the upper and lower surfaces of the slab are neither planar nor smooth (that is, have poor "surface accuracy"). Such a slab is unacceptable in commerce because its surface contains cracks, or, provides locations from which cracks can propagate when the slab is rolled. A slab with poor surface accuracy is evidence of the "casting problem"--the less accurate the surface, the greater the problem.
As indicated above, the design and construction of a block caster derives from a fundamental decision whether to restrain the forces of distortion by equal and opposite restraints, or, to control and limit the distortions without substantially restraining them, and to cope with the controlled distortion.
We decided that the first step towards solving the well-recognized problem of poor surface accuracy was to provide a machine designed to allow the mold blocks to undergo their cyclical thermal changes without substantially restraining them, yet without interfering with the continuously planar configuration of the opposed surfaces of the mold cavity. To this end we provided barely visible slits (referred to as "microslits") of critical width and depth relative to the dimensions of a mold block, cut (in the casting direction, or at an acute angle thereto) in the face of each mold block.
Despite the microslits, the distortion of the blocks was sufficient to require accommodating their displacement in the casting direction. Such accomodate, along with the requirement for sufficient tolerance between blocks to enable insertion of the last block in each casting train, resulted in relatively large "play" between next-adjacent blocks.
Such "play" is closely related to the foregoing problem of poor surface accuracy which is exacerbated by the high sensitivity of the casting, especially in its semi-molten state, to mechanical excitation, referred to as "mechanical noise". By mechanical noise we more particularly refer to a periodicity of events due to any periodic structural excitation as evidenced by a change in the mechanical properties of the casting. An example of such noise is provided by the vibrations transmitted to the mold blocks during operation. The level of vibration which at least in part generates such mechanical noise may be measured by an accelerometer. The severity of the contribution of such excitation has not been recognized, therefore not addressed in the prior art.
Even a relatively low level of mechanical noise is highly undesirable, yet inexplicably, the problem of nullifying its effects, appears never to have been successfully solved to our knowledge, even accidentally, in prior art continuous casting machines.
Evidence of such non-recognition of the effects of mechanical noise is provided in diagrammatic illustrations of numerous prior art continuous casting machines in which the faces of mold blocks in each carriage track, in or near the open ends of the mold cavity, are no longer contiguous. These diagrams show, in the "bends" of each continuous carriage track, that adjacent rectangular mold blocks have radially divergent corners, and circumferentially spaced-apart faces. When the mold blocks fall together due to gravity, one atop the other, the acceleration and immediate deceleration cause a succession of collisions. Such collisions (familiarly referred to as "bangs") occur near the bottoms of the bends in each carriage track. The desired geometry of the mold cavity is restored as each next-adjacent block is subjected to impact, but the mechanical noise so caused disrupts the solidification of molten metal throughout the casting.
We decided the second step was to minimize mechanical noise. We decided to provide interconnected mold block assemblies which would go around the bends without colliding with ("banging") one another.
Assuming one could provide a solution to each step of the problem, one must ensure continuous operation of the machine under enormous forces generated by exigent temperature conditions. Eventually, the forces generate surface defects in the mold faces which require that the mold blocks be replaceable. The more prone the blocks to permanent distortions, the more frequently they need to be replaced.
In the prior art, specifically the '586 Lauener machine, this need to replace mold blocks has dictated that they be endlessly connected so that each mold block assembly is rotatably disposed on a guide roll axle on only one side of the assembly, the other side being held by a guide member which permits both pivotable movement of the mold block and linear movement of the mold block in the casting direction. Thus interconnected in the prior art carriage tracks, each block is individually articulated so that it is pivotable about an axle in which a guide roll axle is journaled on one side, but slidably detachable from the guide roll axle of the next-adjacent mold block assembly. Though interconnected, the prior art mold block assemblies are not interlinked.
The combined movements, due to expansion and contraction, of separably interconnected, individually mounted but not interlinked mold block assemblies are insufficient to relieve the distortions of the mold blocks adequately, as is evident from the massive restraints required in the Lauener '586 machine.
Of far greater consequence is that the articulation of the mold blocks, as provided in the prior art, results in the mold block assemblies striking each other ("banging") as they necessarily accelerate around the bends, causing in turn, a high level of mechanical noise, sufficient to have a deleterious effect on the metal cast.
The inevitability of such banging action is readily deduced from illustrations of block casters in U.S. Pat. No. 3,570,586 to Lauener (class 164/subclass 430); U.S. Pat. No. 3,747,666 to Gyongyos (class 164/subclass 279); and U.S. Pat. No. 4,895,202 to Sato et al (class 164/subclass 340), inter alia.
As described in detail herebelow, the mold block assemblies of our invention are interlinked one to another, as if in a chain (hence referred to as "chain-wise linking"), which if opened to separate the ends of the chain, allows one to hold all the mold blocks supported by only the first at one end of the chain.
It occurred to us that the key to minimizing such mechanical noise lay in the chain-wise interlinking of mold block assemblies with elastic hinge means, so that the faces of the mold blocks were always essentially contiguous. By "essentially contiguous" we mean that if they are spaced apart ("gapped"), the gap is less than 0.050" (inch), preferably less than 0.020". In such a configuration, the faces of the mold blocks form a smoothly continuous arcuate surface in the bends. If the mold blocks could be biased against each other at all times, it seemed unlikely that there would be much mechanical noise. However, it would be impractical to insert the last mold block assembly into a loop, because of the inherent spatial restriction, unless the elasticized casting train permitted it.
Nor, unless the elasticized casting train permitted it, would chain-wise interlinking of the blocks permit compensation for the block-to-block centerline distances which would necessarily vary depending upon the instantaneous position of the block in a loop (kinematic considerations and machine tolerances) and the block's thermal condition at that moment (thermally induced dimensional variation).
Since the foregoing dictated that the spaced-apart relationship of every mold block in each loop be finitely controllable, it was evident that chain-wise interlinking of the mold blocks, if it could be done at all, was impractical unless the casting train could be elasticized.
Still further, we deduced that mechanical noise could be minimized by providing each loop with at least four arcs, with the radius of each being different.
Concisely stated, the problem with prior art continuous casting machines is that they were unable to dampen the mechanical excitation caused by the mold blocks banging against each other in the bends. Neither could the prior art mold block assemblies accommodate the thermal stresses to which the mold blocks were subject. This resulted in castings with cracked or otherwise striated surfaces, and unreliable operation of the machine. The mold block assemblies of the prior art were separately articulated (not chain-wise inter-linked) to enable easy replacement of damaged mold blocks; and each casting train was guided in conventional guide tracks (or roller-ways) which contributed to the problem of mechanical noise.
The solution to the problem of poor surface accuracy of a cast slab is provided in the present invention by chain-wise interlinking of the mold block assemblies in an elastic casting train; providing pairs of rollers in off-set roller ways; providing asymmetric loops to reduce the net effects of excitation in the bends by maintaining the inputs from positive and negative block acceleration out of phase; and, using micro-slitted mold block faces to relieve thermal stresses.